What is considered a reputable print source?
Great question! (I wish more people asked this.) I think the two most important things to consider when deciding that a print source is reputable are "Who wrote it?" and "How can I confirm what they are saying?"
Here's an easy, cut-and-dry example: A blog post written by a mysterious or unqualified author, with no citations/footnotes/quotes and with clear bias is probably a disreputable source. An article written by a professor at a university or by a real journalist, with lots of citations and footnotes, published by a journal or publication with a long history of publishing is probably a reputable source. Here's an example of a terrible source. Here's an example of a reputable source.
Some reputable sources: national newspapers (New York Times; Washington Post); large, popular magazines (Time; Newsweek); scholarly journals (peer-reviewed); and academic books (most of our non-fiction books in Gorgas are written by academics and scholars).
Some sources to avoid: newspapers and magazines with obvious political leanings; smaller newspapers and magazines, which might not go through rigorous editing; books published by smaller publishers or that include bad/no research or citations.
I hope this helps -- the answer is not always so clear. If you would like help more specific to your assignment and its sources, I'd be happy to point you in the right direction.